Process for separating 2,4-toluene diisocyanate from isomers of toluene diisocyanate

ABSTRACT

This invention comprises a process for separating 2,4-toluene diisocyanate from a feed mixture with 2,6-toluene diisocyanate. The process comprises contacting the mixture at adsorption conditions with an adsorbent comprising a Y-type zeolite cation exchanged with a cation in the group Na, Ca, Li or Mg, thereby selectively adsorbing the 2,4-toluene diisocyanate. The remainder of the feed mixture is removed from the adsorbent and the adsorbed toluene diisocyanate isomer is recovered by desorption at desorption conditions with a desorbent material comprising toluene.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of U.S. Ser. No. 915,826,filed Oct. 6, 1986 now abandoned, which is a continuation of U.S. Ser.No. 781,561, filed Sept. 30, 1985, now abandoned.

FIELD OF THE INVENTION

The field of art to which this invention pertains is the solid bedadsorptive separation of isomers of toluene diisocyanate. Morespecifically, the invention relates to a process for separating2,4-toluene diisocyanate from the other toluene diisocyanate isomers byemploying a solid bed adsorption system.

BACKGROUND INFORMATION

The isomers, 2-4-toluene diisocyanate and 2,6-toluene diisocyanate areimportant starting materials for making polyurethanes which are usefulin many applications as rigid or flexible forms or as fibers, e.g.,insulation, soundproofing, interlinings for clothing and sleeping bags,cushions, spandex, etc.

It is common industrial practice to make polyurethane from a mixture ofthe isomers, 2,4- and 2,6-toluene diisocyanate (TDI), for example 80/20or 65/35, derived from 2,4- and 2,6-toluene diisocyanate, because it isdifficult and expensive to separate them by existing techniques. Currentmethods of separating the isomers involve crystallization and hence, aretime-consuming. Moreover, polyurethanes synthesized from the pure 2,4-and 2,6-toluene diisocyanate have dramatically different propertiescompared to materials synthesized from mixtures. I have found that thetensile strength of polyurethane made of 2,4-toluene diisocyanate isgreater than that of 2,6-toluene diisocyanate or any blend of thetoluene diisocyanates and, in fact, increases as the level of2,4-toluene diisocyante increases. Accordingly, it is desirable toseparate the TDI isomers by an economical process.

It is well known in the separation art that certain crystallinealuminosilicates can be used to separate hydrocarbon types from mixturesthereof. Furthermore, X and Y zeolites have been employed in a number ofprocesses to separate individual hydrocarbon isomers.

It is known from U.S. Pat. No. 3,069,470 to Fleck et al, to use type Xzeolites for the separation of the meta isomer from other isomers oftoluidine. From U.S. Pat. No. 4,480,129, it is known that X and Y typezeolites, exchanged with transition metals, are paraselective in amixture of isomers of toluidine.

U.S. Pat. No. 4,061,662 discloses the adsorption of unreacted toluenediisocyanate from polyisocyanate with X-type zeolites. U.S. Pat. No.4,169,175 discloses removal of less than 0.7% unreacted toluenediisocyanate from urethane prepolymers with X-type zeolites.

Yabroff U.S. Pat. No. 4,246,187 discloses a method for separating the2,4- and 2,6- isomers of toluene diisocyanate involving steps ofcrystallizing and centrifuging.

In U.S. Pat. No. 3,575,820, it is disclosed that ortho isomers oftoluene diisocyanate can be removed from toluene diisocyanate mixturesby incorporating aluminum oxide which will polymerize the ortho isomers,whereupon the non-vicinal isomers can be separated by distillation.Chemical Abstract 101:116099X (1984) discloses a treatment for removingtoluene diisocyanate from waste gas by adsorption with activated carbon.

In Japanese Patent Application No. 56905/79, publicly disclosed on Nov.20, 1980, it is disclosed that a solid adsorbent containing titaniumoxide will selectively adsorb the para-isomer of toluidine.

It is known from U.S. Pat. No. 4,270,013 to Priegnitz et al thatortho-nitrotoluene may be separated from other nitrotoluene isomers byusing a type-X zeolite containing at exchangeable cationic sites onecation selected from a group that includes potassium and barium. Thespecific desorbent materials disclosed by this reference are toluene and1-hexanol. The separation of isomers of di-substituted benzenes withcrystalline aluminosilicates having silica/alumina mole ratio of atleast 12 is disclosed in my U.S. Pat. No. 4,467,126.

SUMMARY OF THE INVENTION

In brief summary, the invention is, in one embodiment, a process forseparating 2,4-toluene diisocyanate from a mixture comprising2,4-toluene diisocyanate and at least one isomer thereof, such as2,6-toluene diisocyanate. The process comprises contacting the mixtureat adsorption conditions with an adsorbent comprising a Y-type zeolitecation exchanged with a cation in the group Ca, Na, Li or Mg, therebyselectively adsorbing the 2,4-toluene diisocyanate thereon. Theremainder of the feed mixture is then removed from the adsorbent and theadsorbed isomer recovered by desorption at desorption conditions with adesorbent material comprising toluene.

Other embodiments of our invention encompass details about feedmixtures, adsorbents, desorbent materials and operating conditions, allof which are hereinafter disclosed in the following discussion of eachof the facets of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing the chromatographic separation of the 2,4- and2,6-isomers of toluene diisocyanate by a Na-Y zeolitic adsorbent.

FIG. 2 is a plot showing the chromatographic separation of the sameisomers by a Ca-Y zeolitic adsorbent.

FIG. 3 is a plot showing the chromatographic separation of the sameisomers by a Li-Y zeolitic adsorbent.

FIG. 4 is a plot showing the chromatographic separation of the sameisomers by a Na-Y zeolitic adsorbent.

DESCRIPTION OF THE INVENTION

At the outset, the definitions of various terms used throughout thespecification will be useful in making clear the operation, objects andadvantages of our process.

A "feed mixture" is a mixture containing one or more extract componentsand one or more raffinate components to be separated by our process. Theterm "feed stream" indicates a stream of a feed mixture which passes tothe adsorbent used in the process.

An "extract component" is a compound or type of compound that is moreselectively adsorbed by the adsorbent while a "raffinate component" is acompound or type of compound that is less selectively adsorbed. In thisprocess, 2,4-toluene diisocyanate is an extract component and2,6-toluene diisocyanate is a raffinate component when the adsorbent isa Na-, Ca-, Li- or Mg-exchanged Y zeolite. In other cases, using otherexchange ions, e.g., potassium, the roles are reversed, with the2,6-toluene diisocyanate becoming the extract and 2,4-toluenediisocya.nate becoming a raffinate component. The term "desorbentmaterial" shall mean generally a material capable of desorbing anextract component. The term "desorbent stream" or "desorbent inputstream" indicates the stream through which desorbent material passes tothe adsorbent. The term "raffinate stream" or "raffinate output stream"means a stream through which a raffinate component is removed from theadsorbent. The composition of the raffinate stream can vary fromessentially 100% desorbent material to essentially 100% raffinatecomponents. The "extract stream" or "extract output stream" shall mean astream through which an extract material which has been desorbed by adesorbent material is removed from the adsorbent. The composition of theextract stream, likewise, can vary from essentially 100% desorbentmaterial to essentially 100% extract components. At least a portion ofthe extract stream and, preferably, at least a portion of the raffinatestream from the separation process are passed to separation means,typically fractionators, where at least a portion of the desorbentmaterial is separated to produce an extract product and a raffinateproduct. The terms "extract product" and "raffinate product" meanproducts produced by the process containing, respectively, an extractcomponent and a raffinate component in higher concentrations than thosefound in the extract stream and the raffinate stream. Although it ispossible by the process of this invention to produce a high purityproduct at high recoveries, it will be appreciated that an extractcomponent is never completely adsorbed by the adsorbent, nor is araffinate component completely nonadsorbed by the adsorbent. Therefore,varying amounts of a raffinate component can appear in the extractstream and, likewise, varying amounts of an extract component can appearin the raffinate stream. The extract and raffinate streams then arefurther distinguished from each other and from the feed mixture by theratio of the concentrations of an extract component and a raffinatecomponent appearing in the particular stream. More specifically, theratio of the concentration of the more selectively adsorbed isomer tothat of the less selectively adsorbed isomer will be lowest in theraffinate stream, next highest in the feed mixture, and the highest inthe extract stream.

The term "selective pore volume" of the adsorbent is defined as thevolume of the adsorbent which selectively adsorbs an extract componentfrom the feed mixture. The term "non-selective void volume" of theadsorbent is the volume of the adsorbent which does not selectivelyretain an extract component from the feed mixture. This volume includesthe cavities of the adsorbent which contain no adsorptive sites and theinterstitial void spaces between adsorbent particles. The selective porevolume and non-selective void volume are generally expressed involumetric quantities and are of importance in determining the properflow rates of fluid required to be passed into an operational zone forefficient operations to take place for a given quantity of adsorbent.When adsorbent "passes" into an operational zone employed in oneembodiment of this process, its non-selective void volume together withits selective pore volume carries fluid into that zone. Thenon-selective void volume is utilized in determining the amount of fluidwhich should pass into the same zone in a countercurrent direction tothe adsorbent to displace the fluid present in the nonselective voidvolume. If the fluid flow rate passing to a zone is smaller than thenon-selective void volume rate of adsorbent material passing into thatzone, there is a net entrainment of liquid into the zone by theadsorbent. Since this net entrainment is a fluid present in thenon-selective void volume of the adsorbent, it in most instancescomprises less selectively retained feed components. The selective porevolume of an adsorbent can in certain instances adsorb portions ofraffinate material from the fluid surrounding the adsorbent since incertain instances there is competition between extract material andraffinate material for adsorptive sites within the selective porevolume. If a large quantity of raffinate material with respect toextract material surrounds the adsorbent, raffinate material can becompetitive enough to be adsorbed by the adsorbent.

The prior art has recognized that certain characteristics of adsorbentsare highly desirable, if not absolutely necessary, to the successfuloperation of a selective adsorption process. Such characteristics areequally important to this process. Among such characteristics are:adsorptive capacity for some volume of an extract component per volumeof adsorbent; the selective adsorption of an extract component withrespect to a raffinate component and the desorbent material; andsufficiently fast rates of adsorption and desorption of an extractcomponent to and from the adsorbent. Capacity of the adsorbent foradsorbing a specific volume of an extract component is, of course, anecessity; without such capacity the adsorbent is useless for adsorptiveseparation. Furthermore, the higher the adsorbent's capacity for anextract component the better is the adsorbent. Increased capacity of aparticular adsorbent makes it possible to reduce the amount of adsorbentneeded to separate an extract component of known concentration containedin a particular charge rate of feed mixture. A reduction in the amountof adsorbent required for a specific adsorptive separation reduces thecost of a separation process. It is important that the good initialcapacity of the adsorbent be maintained during actual use in theseparation process over some economically desirable life.

The second necessary adsorbent characteristic is the ability of theadsorbent to separate components of the feed; or, in other words, thatthe adsorbent possess adsorptive selectivity, (B), for one component ascompared to another component. Relative selectivity can be expressed notonly for one feed component as compared to another but can also beexpressed between any feed mixture component and the desorbent material.The selectivity, (B), is defined as the ratio of the two components ofthe adsorbed phase over the ratio of the same two components in theunadsorbed phase at equilibrium conditions. Relative selectivity isshown as Equation 1, below: ##EQU1## where C and D are two components ofthe feed represented in volume percent and the subscripts A and Urepresent the adsorbed and unadsorbed phases respectively. Theequilibrium conditions were determined when the feed passing over a bedof adsorbent did not change composition after contacting the bed ofadsorbent. In other words, there was no net transfer of materialoccurring between the unadsorbed and adsorbed phases. Where selectivityof two components approaches 1.0, there is no preferential adsorption ofone component by the adsorbent with respect to the other; they are bothadsorbed (or non-adsorbed) to about the same degree with respect to eachother. As the (B) becomes less than or greater than 1.0, there is apreferential adsorption by the adsorbent for one component with respectto the other. When comparing the selectivity by the adsorbent of onecomponent C over component D, a (B) larger than 1.0 indicatespreferential adsorption of component C within the adsorbent. A (B) lessthan 1.0 would indicate that component D is preferentially adsorbedleaving an unadsorbed phase richer in component C and an adsorbed phasericher in component D.

The third important characteristic is the rate of exchange of theextract component of the feed mixture material or, in other words, therelative rate of desorption of the extract component. Thischaracteristic relates directly to the amount of desorbent material thatmust be employed in the process to recover the extract component fromthe adsorbent; faster rates of exchange reduce the amount of desorbentmaterial needed to remove the extract component and therefore permit areduction in the operating cost of the process. With faster rates ofexchange, less desorbent material has to be pumped through the processand separated from the extract stream for reuse in the process.

Adsorbents to be used in the process of this invention will comprisespecific crystalline aluminosilicates. Particular crystallinealuminosilicates encompassed by the present invention includecrystalline aluminosilicate cage structures in which the alumina andsilica tetrahedra are intimately connected in an open three-dimensionalnetwork to form cage-like structures with window-like pores of about 8 Åfree diameter. The tetrahedra are crosslinked by the sharing of oxygenatoms with spaces between the tetrahedra occupied by water moleculesprior to partial or total dehydration of this zeolite. The dehydrationof the zeolite results in crystals interlaced with cells havingmolecular dimensions and thus the crystalline aluminosilicates are oftenreferred to as "molecular sieves", particularly when the separationwhich they effect is dependent essentially upon differences between thesizes of the feed molecules as, for instance, when smaller normalparaffin molecules are separated from larger isoparaffin molecules byusing a particular molecular sieve.

In hydrated form, the crystalline aluminosilicates used in the processof this invention generally encompass those zeolites represented by theFormula 1 below:

Formula 1

    M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O

where "M" is a cation which balances the electrovalence of thealuminumcentered tetrahedra and which is generally referred to as anexchangeable cationic site, "n" represents the valence of the cation,"w" represents the moles of SiO₂, and "y" represents the moles of water.The generalized cation "M" may be monovalent, divalent or trivalent ormixtures thereof.

The prior art has generally recognized that adsorbents comprising X andY zeolites can be used in certain adsorptive separation processes. Thesezeolites are described and defined in U.S. Pat. Nos. 2,882,244 and3,130,007, respectively, incorporated herein by refernce thereto. The Yzeolite in the hydrated or partially hydrated form can be represented interms of mole oxides as in Formula 2 below:

Formula 2

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O

where "M" is at least one cation having a valence not more than 3, "n"represents the valence of "M", "w" is a value greater than about 3 up toabout 6, and "y" is a value up to about 9 depending upon the identity of"M" and the degree of hydration of the crystal. The SiO₂ /Al₂ O₃ moleratio for Y zeolites can thus be from about 3 to about 6. The cation "M"may be one or more of a variety of cations such as a hydrogen cation, analkali metal cation, an alkaline earth cation or other selected cations,and is generally referred to as an exchangeable cationic site but, asthe Y zeolite is initially prepared, the cation "M" is usuallypredominately sodium. A Y zeolite containing predominately sodiumcations at the exchangeable cationic sites is referred to as a sodium-Yzeolite.

Cations occupying exchangeable cationic sites in the zeolite may bereplaced with other cations by ion exchange methods well known to thosehaving ordinary skill in the field of crystalline aluminosilicates. Suchmethods are generally performed by contacting the zeolite or anadsorbent material containing the zeolite with an aqueous solution ofthe soluble salt of the cation or cations desired to be placed upon thezeolite. After the desired degree of exchange takes place, the sievesare removed from the aqueous solution, washed, and dried to a desiredwater content. By such methods the sodium cations and any non-sodiumcations which might be occupying the exchangeable sites as impurities ina zeolite can be partially or essentially completely replaced with othercations. The zeolite used in the process of this invention containscations at exchangeable cationic sites selected from the group of metalsNa, Ca, Li or Mg.

Typically, adsorbents used in separative processes contain zeolitecrystals dispersed in an amorphous material or inorganic matrix. Thezeolite will typically be present in the adsorbent in amounts rangingfrom about 75 to about 98 wt. % based on volatile-free composition.Volatile-free compositions are generally determined after the adsorbenthas been calcined at 9° C. in order to drive off all volatile matter.The remainder of the adsorbent will generally be the inorganic matrixmaterial such as silica, titania, or alumina or mixtures thereof, orcompounds, such as clays, which material is present in intimate mixturewith the small particles of the zeolite material. This matrix materialmay be an adjunct of the manufacturing process for zeolite (for example,intentionally incomplete purification of either zeolite during itsmanufacture) or it may be added to relatively pure zeolite, but ineither case its usual purpose is as a binder to aid in forming oragglomerating the hard crystalline particles of the zeolite. Normally,the adsorbent will be in the form of particles such as extrudates,aggregates, tablets, macrospheres or granules having a desired particlesize range. The typical adsorbent will have a particle size range ofabout 16-60 mesh (Standard U.S. Mesh). An example of a zeolite used inadsorbents known to the art, either as is or after cation exchange, is"SK-40", available from the Linde Company, Tonawanda, New York. "SK-40"contains Y zeolite.

Ideally, desorbent materials should have a selectivity equal to about 1or slightly less than 1 with respect to all extract components so thatall of the extract components can be desorbed as a class with reasonableflow rates of desorbent material and so that extract components candisplace desorbent material in a subsequent adsorption step. Whileseparation of an extract component from a raffinate component istheoretically possible when the selectivity of the adsorbent for theextract component with respect to the raffinate component is justslightly greater than 1.0, it is preferred that such selectivity bereasonably greater than 1.0. Like relative volatility, the higher theselectivity, the easier the separation is to perform. Higherselectivities permit a smaller amount of adsorbent to be used.

Desorbent materials used in various prior art adsorptive separationprocesses vary depending upon such factors as the type of operationemployed. In the swing bed system in which the selectively adsorbed feedcomponent is removed from the adsorbent by a purge stream desorbent,selection is not as critical and desorbent materials comprising gaseoushydrocarbons such as methane, ethane, etc., or other types of gases suchas nitrogen or hydrogen may be used at elevated temperatures or reducedpressures or both to effectively purge the adsorbed feed component fromthe adsorbent. However, in adsorptive separation processes which aregenerally operated continuously at substantially constant pressures andtemperatures to insure liquid phase, the desorbent material must bejudiciously selected to satisfy many criteria. First, the desorbentmaterial should displace an extract component from the adsorbent withreasonable mass flow rates without itself being so strongly adsorbed asto unduly prevent an extract component from displacing the desorbentmaterial in a following adsorption cycle. Expressed in terms of theselectivity (hereafter discussed in more detail), it is preferred thatthe adsorbent be more selective for all of the extract components withrespect to a raffinate component than it is for the desorbent materialwith respect to a raffinate component. Secondly, desorbent materialsmust be compatible with the particular adsorbent and the particular feedmixture. More specifically, they must not reduce or destroy the criticalselectivity of the adsorbent for an extract component with respect to araffinate component. Desorbent materials should additionally besubstances which are easily separable from the feed mixture that ispassed into the process. Both the raffinate stream and the extractstream are removed from the adsorbent in admixture with desorbentmaterial and without a method of separating at least a portion of thedesorbent material, the purity of the extract product and the raffinateproduct would not be very high nor would the desorbent material beavailable for reuse in the process. It is therefore contemplated thatany desorbent material used in this process will preferably have asubstantially different average boiling point than that of the feedmixture to allow separation of at least a portion of desorbent materialfrom feed components in the extract and raffinate streams by simplefractional distillation, thereby permitting reuse of desorbent materialin the process. The term "substantially different" as used herein shallmean that the difference between the average boiling points between thedesorbent material and the feed mixture shall be at least about 5° C.The boiling range of the desorbent material may be higher or lower thanthat of the feed mixture. Finally, desorbent materials should also bematerials which are readily available and therefore reasonable in cost.In the preferred isothermal, isobaric, liquid-phase operation of theprocess of our invention, we have found that toluene will result inselectivity for the 2,4-toluene diisocyanate when a Y zeolite isexchanged for Na, Li, Ca and Mg.

The adsorbent may be employed in the form of a dense compact fixed bedwhich is alternatively contacted with the feed mixture and desorbentmaterials. In the simplest embodiment of the invention, the adsorbent isemployed in the form of a single static bed in which case the process isonly semi-continuous. In another embodiment, a set of two or more staticbeds may be employed in fixed bed contacting with appropriate valving sothat the feed mixture is passed through one or more adsorbent beds whilethe desorbent materials can be passed through one or more of the otherbeds in the set. The flow of feed mixture and desorbent materials may beeither up or down through the desorbent. Any of the conventionalapparatus employed in static bed fluid-solid contacting may be used.

Moving bed or simulated moving bed flow systems, however, have a muchgreater separation efficiency than fixed bed systems and are thereforepreferred. In the moving bed or simulated moving bed processes, theretention and displacement operations are continuously taking placewhich allows both continuous production of an extract and a raffinatestream and the continual use of feed and displacement fluid streams. Onepreferred embodiment of this process utilizes what is known in the artas the simulated moving bed countercurrent flow system. The operatingprinciples and sequence of such a flow system are described in U.S. Pat.No. 2,985,589 to D. B. Broughton, incorporated herein by refernce. Insuch a system, it is the progressive movement of multiple liquid accesspoints down a molecular sieve chamber that simulates the upward movementof molecular sieve contained in the chamber. Reference can also be madeto a paper entitled "Continuous Adsorptive Processing - A New SeparationTechnique" by D. B. Broughton presented at the 34th Annual Meeting ofthe Society. of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969,incorporated herein by reference, for further explanation of thesimulated moving bed countercurrent process flow scheme.

Another embodiment of a simulated moving bed flow system suitable foruse in the process of the present invention is the cocurrent highefficiency simulated moving bed process disclosed in our assignee's U.S.Pat. No. 4,402,832, incorporated by reference herein in its entirety.

It is contemplated with any flow scheme used to carry out the presentinvention that at least a portion of the extract output stream will passinto a separation means wherein at least a portion of the desorbentmaterial can be separated to produce an extract product containing areduced concentration of desorbent material. Preferably, but notnecessary to the operation of the process, at least a portion of theraffinate output stream will also be passed to a separation meanswherein at least a portion of the desorbent material can be separated toproduce a desorbent material stream which can be reused in the processand a raffinate product containing a reduced concentration of desorbentmaterial. The separation means will typically be a fractionation column,the design and operation of which is well known to the separation art.

Although both liquid and vapor phase operations can be used in manyadsorptive separation processes, liquid-phase operation is preferred forthis process because of the lower temperature requirements and becauseof the higher yields of extract product that can be obtained withliquid-phase operation over those obtained with vapor-phase operation.Desorption conditions will thus include, as hereinbefore mentioned, apressure sufficient to maintain liquid-phase. Adsorption conditions willinclude the same range of temperatures and pressures as used fordesorption conditions.

A static test procedure and apparatus may be employed to test variousadsorbents with a particular feed mixture to determine the relativeretention by the adsorbent of each component of the mixture. Theprocedure involves mixing together equal quantities of each component,the relative retention of which is to be determined, and a convenientsolvent or desorbent material. A desorbent is selected that will have aboiling point well separated from those of the isomers being tested. Theresulting solution is then placed in a vessel with a quantity of theappropriate adsorbent and is allowed to remain, with occasionalstirring, for about 24 hours. The solution is then analyzed for eachcomponent and the relative retention thereof is determined in terms ofthe ratio, R, of the less strongly adsorbed component to the morestrongly adsorbed component, the relative retention of the more stronglyretained component being greater, the higher the above ratio.

A dynamic testing apparatus is employed to test various adsorbents witha particular feed mixture and desorbent material to measure theadsorption characteristics of retention capacity and exchange rate. Theapparatus consists of a helical adsorbent chamber of approximately 70-80cc volume having inlet and outlet portions at opposite ends of thechamber. The chamber is contained within a temperature control meansand, in addition, pressure control equipment is used to operate thechamber at a constant predetermined pressure. Quantitative andqualitative analytical equipment such as refractometers, polarimetersand chromatographs can be attached to the outlet line of the chamber andused to detect quantitatively or determine qualitatively one or morecomponents in the effluent stream leaving the adsorbent chamber. A pulsetest, performed using this apparatus and the following generalprocedure, is used to determine data for various adsorbent systems. Theadsorbent is filled to equilibrium with a particular desorbent materialby passing the desorbent material through the adsorbent chamber. At aconvenient time, a pulse of feed containing known concentrations of atracer and of a particular extract component or of a raffinate componentor both, all diluted in desorbent material is injected for a duration ofseveral minutes. Desorbent material flow is resumed, and the tracer andthe extract component or the raffinate component (or both) are eluted asin a liquid-solid chromatographic operation. The effluent can beanalyzed on-stream or alternatively, effluent samples can be collectedperiodically and later analyzed separately by analytical equipment andtraces of the envelopes or corresponding component peaks developed.

From information derived from the test, adsorbent performance can berated in terms of void volume, retention volume for an extract or araffinate component, and the rate of desorption of an extract componentfrom the adsorbent. The retention volume of an extract or a raffinatecomponent may be characterized by the distance between the center of thepeak envelope of the extract or raffinate component and the center ofthe peak envelope of the tracer component or some other known referencepoint. It is expressed in terms of the volume in cubic centimeters ofdesorbent material pumped during this time interval represented by thedistance between the peak envelopes. The rate of exchange of an extractcomponent with the desorbent material can generally be characterized bythe width of the peak envelopes at half intensity. The narrower the peakwidth, the faster the desorption rate. The desorption rate can also becharacterized by the distance between the center of the tracer peakenvelope and the disappearance of an extract component which has justbeen desorbed. This distance is again the volume of desorbent materialpumped during this time interval.

The following non-limiting examples are presented to illustrate theprocess of the present invention and are not intended to unduly restrictthe scope of the claims attached hereto.

EXAMPLE 1

A number of static tests were performed to demonstrate that it waspossible to separate the isomers by an adsorptive process. In this case,because the analytical method will not completely resolve the isomers ofTDI, the adsorption test was conducted with an analogous compound,phenyl isocyanate. Phenyl isocyanate, which can be analytically resolvedfrom both the 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, isseparately mixed with each of the isomers and 2 sets of tests areconducted for each adsorbent. The relative static selectivity A of theisomers is obtained by first calculating that of phenyl isocyanate witheach of the isomers as follows: ##EQU2## Then,

    A.sub.2,6/2,4 =A.sub.PI/2,4 ×A.sub.2,6/PI

In an inert atmosphere, a stock solution of each tolene diisocyanate(TDI) isomer with phenyl isocyanate and isooctane was made up as followsand tested separately:

    ______________________________________                                        2,4-TDI or 2,6-TDI    5.88 vol. %                                             phenyl isocyanate     5.88 vol. %                                             isooctane             balance                                                 ______________________________________                                    

In all the static tests, the volume ratio of stock solution to adsorbentwas 1.5. The temperature was 25° C. The stock solution and adsorbentwere combined in a flask and the amount of each isomer left in theraffinate was determined and the static selectivity, A, of2,6-TDI/2,4-TDI was calculated in the manner just described for a numberof adsorbents. The results are as follows:

    ______________________________________                                        Adsorbent           A 2,6-/2,4-                                               ______________________________________                                        BaK--X              no adsorption                                             K--Y                1.63                                                      Na--Y               0.79                                                      Li--Y               0.71                                                      Mg--Y               0.60                                                      ______________________________________                                    

These tests show that 2,4-TDI is selectively adsorbed by Na-Y, Li-Y andMg-Y, while 2,6-TDI is selectively adsorbed by K-Y. Hence, these isomersmay be separated by our adsorptive process. BaK-X showed no adsorptionof either isomer. Several of these adsorbents also underwent the pulsetest as described in the next example, confirming the results of thestatic test.

EXAMPLE 2

The previously described pulse test apparatus was used to obtain datafor this example using a sodium exchanged Y zeolite. The liquidtemperature was 153° C. and the flow was up the column at the rate of1.2 cc/min. The feed stream comprised a 2.6 cc pulse of a solutioncontaining 1 cc of a 65/35 mixture of 2,4- and 2,6-TDI, and 0.8 cc ofn-C₁₄ tracer. The column was packed with clay bound adsorbent of 30-60mesh particle size. The desorbent was 100% toluene.

The selectivity (B), as earlier described, was calculated from the traceof the peaks generated for the components. The results of this exampleare shown in FIG. 1. The selectivity, B₂,4/2,6 =1 40.

EXAMPLES 3-4

Further pulse tests were done, using the same conditions as Example 2except that the temperatures and adsorbents were different as set forthin the following table, together with the selectivity (B)₂,4/2,6calculated from the trace of the peaks in FIGS. 2 and 3:

    ______________________________________                                        Example Adsorbent Liq. Temp. B2,4/2,6                                                                              Desorbent                                ______________________________________                                        3       Ca--Y     150° C.                                                                           1.72    toluene                                  4       Li--Y     155° C.                                                                           1.44    toluene                                  ______________________________________                                    

EXAMPLE 5

The previously described pulse test apparatus was used to obtain datafor this example using a sodium exchanged Y zeolite. The liquidtemperature was 100° C. and the flow was down the column at the rate of1.26 cc/min. The feed stream comprised a 2.6 cc pulse of a solutioncontaining 2 cc of a 80/20 mixture of 2,4- and 2,6-TDI, and 0.5 cc ofn-C₁₄ tracer with the balance desorbent. The column was packed with claybound adsorbent of 30-60 mesh particle size. The desorbent was 100%toluene.

The selectivity (B), as earlier described, was calculated from the traceof the peaks generated for the components. The results of this exampleare shown in FIG. 4. The selectivity, B₂,4/2,6 =1.90.

In general, the above data does show that the present invention providesa 2,4-toluene diisocyanate selective system with Na, Ca, Mg and Li-Y-type zeolite adsorbents with adequate selectivities for the commercialuse of the separation of the present invention.

What is claimed is:
 1. A process for separating 2,4-toluene diisocyanatefrom a feed mixture comprising 2,4-toluene diisocyanate and 2,6-toluenediisocyanate, said process comprising contacting said mixture atadsorption conditions with an adsorbent comprising a Y- type zeolite,cation exchanged with a cation from the group Na, Ca, Li and Mg, therebyselectively adsorbing said 2,4-toluene diisocyanate-, removing theremainder of said mixture from said adsorbent, and then recovering saidadsorbed 2,4-toluene diisocyanate by desorption at desorption conditionswith a desorbent material comprising toluene.
 2. The process of claim 1wherein said adsorption and desorption conditions include a temperaturewithin the range of from about 20° C. to about 200° C. and a pressuresufficient to maintain liquid phase.
 3. The process of claim 1 whereinsaid process is effected with a simulated moving bed flow system.
 4. Theprocess of claim 1 wherein said process is effected with a static bedsystem.